1. Field of the Invention
The present invention generally relates to an apparatus for reproducing record information of multiple-layered optical disc, on which the record information is recorded on a plurality of information record layers, and more particularly to a control device in the reproducing apparatus for moving an objective lens in an optical pickup to focus a reading light onto one of the information record layers, which is specified.
2. Description of the Related Art
In a CD (Compact Disc) player for reproducing a CD, as one example of a conventional optical disc, an objective lens is driven by an actuator in an optical pickup to keep a relative distance between the objective lens and the CD constant at a standard value, on the basis of a focus error signal, in order to deal with a so-called surface deflection due to a warp or distortion of the CD.
On the other hand, a so-called DVD is developed as an optical disc of high density recording type. There are various types of recording formats and reading formats for this DVD. As one type of them, there is a multiple-layered optical disc, which has a structure comprising a plurality of information record layers.
When reproducing the multiple-layered optical disc, a control to switch the information record layers to be read becomes necessary, which is not required in the conventional CD player. Namely, a control to move the objective lens in the optical pickup, which is focusing the reading light onto a first information record layer to be presently read, for a distance corresponding to an inter-layer distance between the first information record layer and a second information record layer specified to be nextly read (hereinbelow, it is referred to as an "inter-layer jump"), becomes necessary, so as to focus the reading light onto the second information record layer.
At the time of moving the objective lens in this inter-layer jump, an acceleration signal to accelerate the optical pickup i.e., to generate a driving force in a constant magnitude so as to move the optical pickup is generated, and, after a predetermined time duration, an deceleration signal to decelerate the optical pickup is generated, so that a focus servo is applied to the specified information record layer.
In this case, if the distance between each of the information record layers in the multiple-layered optical disc and the objective lens in the optical pickup is constant (i.e., if the disc is not warped or distorted, and the inter-layer distance within the disc is even), it is possible to apply the focus servo with respect to the specified information record layer in a stable manner at any position on the disc, by generating a constant acceleration signal which is set in advance and by generating, after a constant time duration, a constant deceleration signal.
By the way, the surface deflection is generated in case of the multiple-layered optical disc, since there exists the warp or distortion of the disc, in the same manner as the aforementioned CD. Because of the influence of this surface deflection, the distance between the multiple-layered optical disc and the objective lens is not always constant but is fluctuated or changed in accompaniment with the rotation of the disc.
In order to deal with this surface deflection, the focus servo actuator section for actuating the objective lens is constructed to actuate or drive the objective lens in the direction substantially perpendicular to the surface of the disc, so as to keep the relative distance between the objective lens in the optical pickup and the multiple layered optical disc always constant at a standard value, in a focus close condition.
When the focus servo is changed from this focus close condition to a focus open condition in order to move the objective lens to a position corresponding to the specified information record layer, the driving force which has been applied to follow the surface deflection in the focus close condition is applied to the objective lens as an inertial driving force of the objective lens as it is, in the focus servo actuator section. Thus, the above mentioned inertial driving force due to the surface deflection must be taken into consideration in case that the inter-layer jump is performed so as to focus the reading lightonto the specified information record layer from the information record layer, which is presently being read.
Namely, assuming that the deceleration signal is constant, the following problems may occur. On one hand, if the inertial driving force due to the surface deflection is applied in the direction same as the acceleration direction by the acceleration signal, the deceleration is not enough, so that the focus servo cannot be properly applied with respect to the specified information record layer since the frequency of the focus error signal is too high as compared with the servo band. On the other hand, if the inertial driving force due to the surface deflection is applied in the direction opposite to the acceleration direction by the acceleration signal, the deceleration is in excess, so that the objective lens returns to a position corresponding to the information record layer which has been read. Since such an influence of the surface deflection is different from a position to a position on the disc where the inter-layer jump is performed, the possibility that the inter-layer jump fails is high at the position where the influence of the surface deflection is relatively large.
Further, since the actuated or driven portion including the objective lens in the optical pickup is subjected to an influence of the gravity due to the self weight thereof, it is necessary to control the magnitude (e.g., the pulse height and the pulse width) of the deceleration signal to be an appropriate value and the application timing of the deceleration signal (e.g., the deceleration waiting time) in consideration of the influence of the gravity depending upon the jump direction.
Furthermore, it is practically impossible, by the conventional mass-production of the multiple-layered optical discs, to limit the inter-layer distances in all of the multiple layered optical discs to be within a range where they do not cause the above mentioned problems at the time of performing the inter-layer jump for respective one of the discs.
In other wards, unevenness of the inter layer distances of the information record layers is inevitable between the discs. Thus, according to the DVD specification, in case of the multiple-layered optical disc, the inter-layer distances of the information record layers are prescribed in a range between 40 to 60 .mu.m (in one disc, the inter-layer distance is designed to be substantially same at any position of the disc).
Thus, since the inter layer distance between the information record layers of the multiple-layered optical disc loaded to the DVD player is not always constant, the inter-layer distance as well as the aforementioned influence of the surface deflection and the jump direction must be taken into consideration at the time of performing the inter-layer jump. Namely, in the example of the above mentioned DVD, there may be a case where the disc having the inter-layer distance of 40 .mu.m is loaded to the DVD player, and a case where the disc having the inter-layer distance of 60 .mu.m is loaded to the DVD player.
Therefore, a problem is raised that the focus servo cannot be appropriately applied to always focus the reading light onto the specified information record layer, unless the surface deflection of the disc and the inter-layer distance between the information record layers are not taken into consideration, at the time of moving the objective lens in the optical pickup in accompaniment with switching the information record layers to be read.